In an automotive vehicle, a member to be assembled to a vehicle body, such as a side member of a subframe for supporting a suspension arm, and a member constituting the vehicle body, such as a front side frame or a rear side frame, are required to be increased in strength from a viewpoint of ensuring strength of the vehicle body to provide enhanced safety, and reduced in weight from a viewpoint of enhancing a fuel efficiency of an engine. For this reason, these members are commonly composed of a closed cross-section structural member made of a metal material.
Heretofore, such a closed cross-section structural member has been produced by: preparing two cross-sectionally hat-shaped metal workpieces each having a base wall, a pair of standing walls each standing from a respective one of widthwise opposite edges of the base wall and a pair of flanges each bending outwardly from a respective one of distal edges of the standing walls; and welding each of two pairs of opposed flanges of the metal workpieces together. More specifically, as illustrated in FIG. 9A, after preparing two metal workpieces 101, 102 whose flanges 111, 112 have different widths, the flanges 111, 112 are superposed on (mated to) each other, and, in this state, a lap fillet welding operation based on arc welding or the like is performed along a lateral edge face of the flange 112 having a smaller width.
In the above welded structural member obtained by welding two metal workpieces, the lower metal workpiece 101 serves as a member for positioning the upper metal workpiece 102, so that it is possible to facilitate position alignment in a direction perpendicular to mated surfaces of the flanges 111, 112. On the other hand, it is necessary to provide the flange 111 (112) along a distal edge of a standing wall of each of the metal workpieces 101, 102, causing a problem that an amount of metal workpiece to be used is increased, resulting in increased weight of the welded structural member, and deteriorated fuel efficiency of an automotive vehicle.
In order to cope with this problem, for example, in a welded structural member illustrated in FIG. 9B, after preparing two cross-sectionally U-shaped metal workpieces devoid of the above flanges, one 202 of the workpieces is fitted in the other workpiece 201, and, in this state, a lap fillet welding operation is performed along an edge face of each standing wall of the outer metal workpiece 201. In this method, no flange is provided in the metal workpieces, so that it is possible to reduce an amount of metal workpiece to be used, and thus facilitate weight reduction, as compared to the welded structural member illustrated in FIG. 9A.
Meanwhile, in the above welded structural member, with a view to weight reduction, the metal workpiece can be prepared using a metal workpiece having a small plate thickness. In this case, as the plate thickness becomes smaller, welding operability is more likely to deteriorate, and a hole is more likely to be formed due to the occurrence of rust. In order to cope with this problem, there are some cases where a plated steel plate is used as a metal workpiece for the metal workpieces. However, the use of a plated steel plate leads to a situation where, when a lap fillet welding operation based on arc welding, laser welding or the like is performed to produce a flange mating-type welded structural member as illustrated in FIG. 9A or a fitting-type welded structural member as illustrated in FIG. 9B, a plated metal is vaporized along with the welding operation to generate metal gas. Then, the generated metal gas is likely to mix in a fusion zone around mated surfaces or fitted surfaces of the flanges, and the fusion zone is solidified while the mixed metal gas is not discharged to the outside, resulting in the occurrence of so-called “blowholes”. The blowholes cause insufficiency of strength of the welded structural member. Thus, there is a need to suppress the occurrence of blowholes.
As measures against this problem, JP 2003-136243A describes a production method for the fitting-type welded structural member, wherein a plurality of concave grooves are formed in each standing wall of one of two cross-sectionally U-shaped metal workpieces intermittently along a welding direction. This method can allegedly suppress the occurrence of blowholes by void spaces based on the concave grooves.
However, the method described in JP 2003-136243A involves a problem that the presence of the concave grooves in the metal workpiece imposes restrictions on welding position, and a problem that the occurrence of blowholes still cannot be suppressed in regions where the two metal workpieces are closely fitted with respect to each other.